1. Field of the Invention
The invention relates to an extrusion system with a moulding device, which has die faces against which an extruded object to be fed therethrough is applied, in particular a section having several cavities, which can be arranged after an extrusion and/or another calibrating die in the direction of extrusion, and has a first end face of the calibrating die forming a gap to an extrusion die and/or calibrating die which can be arranged before it in the direction of extrusion and provided with a sealing device to seal off this gap and form a cavity and all terminal edges of the die faces of the calibrating die are arranged in the region of the end face enclosed by the sealing device, wherein the pressure in this cavity, once formed, can be evacuated to a negative pressure below the ambient pressure.
2. The Prior Art
A moulding device for an extrusion system is known from DE 297 16 343 U1 of the same applicant, which has at least one calibrating device comprising in particular several calibrating dies arranged one after another in the direction of extrusion, this calibrating device and/or the calibrating dies being provided with cooling passages through which a tempering medium flows. In addition, the calibrating device has die faces against which the item being fed through is applied as well as a device to form a cavity and provide a seal between the region of the external surface of the item to be fed through and the surrounding pressure which is arranged at least between the extrusion die of the moulding device, which may be a nozzle for example, and the first calibrating die immediately thereafter. However, this sealing device for forming an additional cavity may also be arranged between the first calibrating die and at least one other calibrating die. In the case of this moulding device, the gap between the extrusion die and the first calibrating die is between 1 mm and 15 mm, preferably 3 mm to 6 mm, but this embodiment has not proved to be capable of producing satisfactory calibration results in all applications, particularly with regard to surface properties and mechanical specifications.
Other moulding devices are known in which the plastic strands of sections fed out from an extrusion nozzle, in particular hollow sections or pipes, are calibrated to desired external and internal dimensions and frozen to the desired external dimensions or cooled to the appropriate temperature that will produce the requisite natural rigidity. Moulding devices of this type are known from the book by Walter Michaeli "Extrusionswerkzeuge fur Kunststoffe und Kautschuk" published by Carl Hanser Verlag, Munich/Vienna, 2.sup.nd fully revised and extended edition, in 1991, particularly pages 321 to 329. Page 323 illustrates an external calibration system using a vacuum, in which the extruded material discharged from the extrusion die in the form of a hollow section is fed, at a certain distance after emerging from the nozzle lip of the extrusion die, into a calibrating die, through which the extruded material is fed so that the region of its circumferential external surface lies against the die faces of the calibration tool. The surface of the extruded material lies against the individual die faces of the calibrating die without any clearance, this being achieved by means of air vents, in particular slits, provided in the die faces, which are connected to a vacuum source by means of supply lines. In order to cool the extruded material, the calibrating dies are provided with one or more surrounding passages for a tempering medium and the vacuum in the individual air vents or slits can be increased at an increasing distance from the nozzle lip of the extrusion die. The temperature of the tempering medium is very low as compared with the mass temperature of the extruded section and is approximately 20.degree. C. One or more cooling baths are generally provided adjoining these so-called dry calibrators, in which the sections, being of the requisite rigidity, are cooled to room temperature, partly by applying a vacuum and wetting from spray nozzles or by feeding them through water baths. In many instances, calibrating dies of this type are not capable of producing an adequate surface quality in the resultant extruded objects once the die has been in service for a longer period of time.
Another device for calibrating extrusion moulded sections, in particular extrusion moulded hollow sections, made from thermoplastic materials is known from DE 22 39 746 A, in which a vacuum calibrating device with an integrated cooler is arranged at a distance apart in the direction of extrusion after the extrusion die or spray head of the screw press charging the plastic material. A vacuum calibration tank through which a coolant flows is provided, connected to the vacuum calibrating device and the vacuum calibrating device is so designed that only the outer skin and optionally any external profiled sections are moulded to their final shape in it, along with any fine contours of the section to be calibrated. The residual warmth still prevailing in the section is fed away by the coolant in the vacuum calibration tank. Arranged at a distance from one another in the vacuum calibrating device are individual calibrating orifices, forming cavities between them in the form of vacuum slits, through which the extruded object is drawn against the die faces of the individual calibrating orifices due to the vacuum applied. Using the calibrating device proposed here, it has not always proved possible to produce an extruded item with perfect surface quality for all applications, nor has the device proved efficient over a longer period of service.